The present invention relates to a retreading device for tires in which a tread strip is applied to a carcass, the device comprising a tread feeding device for feeding the tread to the carcass and an extruder for extruding a cushion gum layer, the extruder having two lips defining the extrusion die. The downstream lip projects past the upstream lip in the direction toward the carcass.
Tire re-treading is carried out conventionally by two basic methods:
A. Mold cure retreading where the casing is prepared by applying a blank tread of uncured rubber with a cushion gum layer facing the casing and then curing the thus prepared green tire. The tread pattern is impressed on the blank in the mold and is bonded to the casing by the application of heat and pressure. The mold can receive only one tire at a time. The vulcanization temperature is generally at about 150.degree. C. This method is expensive. PA0 B. Precure retreading where a tread, already cured and patterned in a flat mold or a ring mold and sized to fit the casing, is bonded to the casing by a thin layer of uncured binding or cushion gum that is subsequently cured. This is done at much reduced pressure and temperature (between 95.degree. and 120.degree. C.) in an autoclave, in which generally a number of tires are so treated at the same time. This is far less expensive. PA0 A. in the plane through the center plane of the casing, a curvature, not necessarily completely circular, having in part a radius at least equal to the biggest buffing radius of any tire, so that under a strong approach pressure (pressing force) of the extruder the casing can be deformed and conform to this curve, and PA0 B. in the direction of the rotation of the casing, a chamfer of a depth varying along said curve.
Both methods require the preparation of the casing (carcass), the removal of the worn tread, and a buffing step to generate a uniform surface, curved in a plane through the axis of the tire in the so-called buffing contour or buffing radius. This results in a predetermined degree of roughness to enable the bonding of the new tread to the casing.
Especially in the retreading of truck tires, which represent much bigger unit values than automobile tires, a separate operation of repairing local damage which becomes apparent on the buffed surface has to be carried out. This is done by grinding the damaged area to expose a roughened, but non-ragged surface, an operation known as skiving. Thereafter the "skives", also called "buzz-outs", are filled with heated repair material, which is also mostly cushion gum. This is often done with small extruders the size of hand-tools in a labor-intensive operation requiring much care and attention.
Especially in precure retreading, the binding gum has historically been prepared by a calendering operation as a wide sheet. Processing temperatures commonly do not exceed 80.degree. C. The sheet is cooled and rolled up with intermediate plastic layers to be later cut into widths corresponding to different sizes of tire and to be applied as part of the labor-intensive process of "building" a precured tire.
The binding gum (cushion gum) thus prepared has the disadvantage that due to its compounding, designed to provide a reasonable time of curing in the autoclave, the sheets begin to "age" in storage, i.e., the binding gum already cures to such an extent that it can no longer fullfil its bonding funchion later. By the same token, it is prone to "scorching" in an extrusion operation unless extrusion is carried out under very carefully controlled conditions, especially when the extruded sheet has to be as thin or thinner than the calendered sheet.
This involves extrusion dies having the width of a casing, i.e., approximately between 140 and 350 mm, but of a slot height of less than the 1.2 or 1 mm thickness for the cushion gum and, depending on the swell-factor, considerably less, i.e., down to 0.4 mm. This results in the generation of a high back pressure during the extrusion operation, and it is thus impossible to keep the extrusion temperature in the range considered safe by the rubber suppliers, namely that of the calendering operation where exceeding the prescribed temperature range by 6.degree. C. to 15.degree. C. is already considered critical, depending on the compound.
As the guarantee for proper bonding is a liability of the rubber supplier, an extrusion apparatus for direct application of cushion gum to a buffed casing to be used by the individual retreader has found little support from the rubber suppliers for the aforementioned reasons.
Thus, although there have been direct application cushion gum extruders on the market for some years and also in use by a few retreaders, there has been no widespread application of this technology as yet.
Moreover, for the sake of safety of "green" adhesion, i.e. adhesion before curing, the conventional industrial practice of spraying the buffed and repaired casing surface with rubber solution has been retained even by the few users of direct cushion gum extrusion application.
However, such rubber solution contains volatiles which present a health hazard. Therefore, in some counties of California and one state of the Federal Republic of Germany, the use of such sprays has already been prohibited, and users must phase out this method. Most likely, this measure will spread worldwide.
U.S. Pat. No. 5,162,070 shows a binding or cushion gum extruder for direct extrusion onto a rotating tire casing. The layer of cushion gum basically has to be extruded to gauge--1.2 mm thickness or less--and is pressed onto the crown of the casing with a gentle, elastic force whereby the die radius in the horizontal plane corresponds to the biggest "buffing-radius" occurring in practice. Outside the more or less wide band of applied pressure, the extruded cushion gum which cools very rapidly shrinks onto the outer casing areas and achieves good adhesion without air inclusion.
This principle works even when, for precured treads having thin "wings" or lateral flanks, cushion gum has to be applied 20 or 30 mm down the sidewalls of the casing. The speeds of extrusion are varied for width adjustment, carried out by mechanically moved side plates, and for affecting the thickness of the applied cushion gum layer. For this purpose, the adjustment of the casing rotation may also be used within limits. The adjustments may be interconnected and preprogrammed by a process controller.
European Patent Application 0528683A1 shows a similar apparatus in which a different way of adjusting the width of the extrusion to that of the casing is proposed, namely the rotation of the die head about its longitudinal axis out of the horizontal plane, in which it is situated for accommodating the greatest width of the buffed crown of the casing.
To ensure conformity of the crown of the casing to the radius of the die in all angular positions, the die is provided with a spacer lip contacting the casing at a position which, due to the rotation of the casing, is upstream of the extruded cushion gum. The other lip, at a given small distance from the buffed surface, then shapes the outside of the layer of cushion gum.
The German Patent Application DE 4025459 A1 teaches the spiral winding of a thin and narrow strip of extruded cushion gum onto a buffed casing. The spirally wound strips overlap edgewise which does not reliably avoid air inclusions. This reference shares this feature with the well-known method of strip-winding for building up tread blanks for mold curing, disclosed in German Patent # 1 729 555.
All three of these examples of the prior art operate the cushion gum extruders against small die slots, i.e. in the mode of high back-pressure produced by the die, and therefore in an extrusion temperature range in excess of the operating temperatures of the conventional calendering operation for cushion gum sheet. Also, none of these methods can effect skive-filling.
The French patent No. 7722974 deals with the filling of irregularly worn or damaged spots on unbuffed tires. It does so, however, in the context of applying a tread blank for mold cure retreading.
The means for achieving this is a die head with interchangeable dies, adapted to the desired contour of the tread. The die head is advanced to the crown of the casing without contacting it and spaced at a fixed distance thereto. The rubber is applied by the extruder. The rubber is then shaped from an agglomeration of rubber formed between the die outlet and the casing by rotation of the casing against the downstream lip of the die, the die opening being formed between the rotating casing and the downstream curved die surface. The radial distance between casing and downstream die surface determines the thickness of the blank built up on the casing.
Such machines have found application in mold retreading, although they have, as far as is known, been used only on casings buffed in the conventional way and have served to a limited degree the purpose of skive-filling. One reason for this is that the material filling the skives in this method is tread rubber, compounded for mileage and road-holding, and not the soft and sticky cushion gum normally used. A more important reason is that the pressure generated by the extruder and pushing on the agglomeration of plasticized rubber can freely expand and is not sufficient to ensure penetration of rubber into any but shallow skive holes, which must, moreover, be carefully shaped to allow easy flow of the rubber. In practice, this means that the skiving operator has to exercise judgement as to which skives to fill in the conventional manner and which to leave to the machine to fill. This introduces a factor of uncertainty in quality control which many retreaders find unacceptable.
In this method, the nature of the rubber application has made the joining of the beginning and the end of the extruded after a full rotation of the casing a problem and this joint or "splice" has generally required cleaning-up in an additional operation, involving cutting off excess rubber and hence producing potential waste.
It is furthermore disadvantageous that the thickness of the tread exhibits great fluctuations when the carcass is not exactly centered.
In view of the above, it is an object of the present invention to provide an apparatus for the direct extrusion application of a cushion gum to a buffed casing in a predetermined thickness under temperature conditions (about 80.degree. C.) which approximate those in the calendering operation for conventionally made cushion gum in sheet form.
It is a further object of the invention to provide for an application under such pressure that penetration into the buffed surface is assured, i.e. that, given a clean buffed surface, rubber solution spraying for "green" adhesion is made superfluous, thereby avoiding environmental pollution and saving usually a complete work station.
It is a further object of the invention to reliably achieve skive-filling, thereby avoiding that this step be performed as a separate, labor-intensive, and quality-sensitive manual operation.
It is a further object of this invention to provide a method to be applicable not only for precure retreading where the cushion gum, being more costly than tread rubber, is applied in as thin a layer as possible, but also for mold cure retreading, where the cushion gum or base gum, especially for truck tires, can be less costly than the tread rubber. The cushion gum then can be applied thinner in the center of the crown of the casing with a build-up in thickness towards the shoulders to form a bigger proportion of the rubber applied for retreading as the base of a so-called "cap and base" tread, for achieving cost-saving on material as well as quality advantages.